There are no rules to get the job done. I was surprised having read a couple of books to learn of the salt trick for waterfalls etc (that was used as recently as Phantom Menace). Like yea you could do it in CG or whatever but whats simpler than dropping a bit of salt and shooting it? Genius & simplicity at the same time! Theres some really wacky stuff going on in Kubrick's 2001 and in the Cinefex retrospective you find out that the way they achieved the results were equally as odd.

Bottom line as others have said: As long as it works it doesnt matter how you do it.